There are essentially two types of data memory devices used in computers today, "Nonvolatile" and "Volatile". Common nonvolatile memory devices include well-known Read Only Memory (ROM) devices that include EPROM (erasable programmable ROM) devices, EEPROM (electrically erasable programmable ROM) devices, and Flash EEPROM devices. These nonvolatile memory devices maintain the data stored therein, even when power to the device is removed, thus they are nonvolatile. Volatile memory devices include Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM) devices. RAM devices in the prior art have been used for temporary data storage, such as during data manipulation, since writing data into, and reading data out of, the device is performed quickly and easily. However, a disadvantage of these devices is that they require the constant application of power, such as in the form of a data refresh signal, to refresh and maintain data stored in the memory cells of the chip. Once power supplied to the device is interrupted, the data stored in the memory cells of the chip is lost.
Flash EEPROMs are most commonly used in all of the nonvolatile memory devices. The Flash EEPROM has an advantage of being nonvolatile, but the flash EEPROM has a problem of endurance. Under repeated cycling program/erase operations, the flash EEPROM is exhausted and results in breaking down, so that the flash EEPROM loses the capability of data storage. In addition, the flash EEPROM relative to DRAM and SRAM needs higher threshold voltage and longer time for programming. Therefore, the speed of the computer will be very slow if the flash EEPROM in used as a memory device.
SRAM having fast data access speed and long lifetime in all of the memory devices is suitable for use in the computer, such as in BIOS. However, the SRAM is a volatile memory device. The data stored in the SRAM will be lost if power is interrupted. Hence, an additional nonvolatile memory device is needed, such as a hard disk, to store data before the power is turned off.
If the SRAM and nonvolatile memory device can be combined, the memory device will have both advantages of the SRAM and the nonvolatile memory device, such as fast data access, long lifetime, and data retention, so that a nonvolatile SRAM will be useful and worthy.